Image sensor and method having an anti-reflection stack with films of different refractive indexes

ABSTRACT

An image sensor pixel that includes a photoelectric conversion unit supported by a substrate and an insulator adjacent to the substrate. The pixel includes a light guide that is located within an opening of the insulator and extends above the insulator such that a portion of the light guide has an air interface. The air interface improves the internal reflection of the light guide. Additionally, the light guide and an adjacent color filter are constructed with a process that optimizes the upper aperture of the light guide. These characteristics of the light guide eliminate the need for a microlens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/218,749 filed on Jul. 16, 2008 now U.S. Pat. No. 7,816,614, whichclaims priority to U.S. Provisional Application No. 61/009,454 filed onDec. 28, 2007; U.S. Provisional Application No. 61/062,773 filed on Jan.28, 2008; U.S. Provisional Application No. 61/063,301 filed on Feb. 1,2008 and U.S. Provisional Application No. 61/069,344 filed on Mar. 14,2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject matter disclosed, generally relates to structures andmethods for fabricating solid state image sensors.

2. Background Information

Photographic equipment such as digital cameras and digital camcordersmay contain electronic image sensors that capture light for processinginto still or video images. Electronic image sensors typically containmillions of light capturing elements such as photodiodes.

Solid state image sensors can be either of the charge coupled device(CCD) type or the complimentary metal oxide semiconductor (CMOS) type.In either type of image sensor, photo sensors are formed in a substrateand arranged in a two-dimensional array. Image sensors typically containmillions of pixels to provide a high-resolution image.

FIG. 1A shows a sectional view of a prior art solid-state image sensor 1showing adjacent pixels in a CMOS type sensor, reproduced from U.S. Pat.No. 7,119,319. Each pixel has a photoelectric conversion unit 2. Eachconversion unit 2 is located adjacent to a transfer electrode 3 thattransfers charges to a floating diffusion unit (not shown). Thestructure includes wires 4 embedded in an insulating layer 5. The sensortypically includes a flattening layer 6 that compensates for top surfaceirregularities due to the wires 4.

Light guides 7 are integrated into the sensor to guide light onto theconversion units 2. The light guides 7 are formed of a material such assilicon nitride that has a higher index of refraction than theinsulating layer 5. Each light guide 7 has an entrance that is widerthan the area adjacent to the conversion units 2. The sensor 1 may alsohave a color filter 8 and a microlens 9.

The microlens 9 focuses the light 7 onto the photo photoelectricconversion units 2. As shown in FIG. 1B because of diffraction optics,the microlens 9 can cause diffracted light that propagates to nearbyphotoelectric conversion units and create optical crosstalk and lightloss. The amount of cross-talk increases when there is a flatteninglayer because the lens is farther away from the light guide. Metalshields are sometimes integrated into the pixels to block cross-talkinglight. The formation, size, and shape of the microlens can be varied toreduce crosstalk. However, extra cost must be added to the precisemicrolens forming process, and crosstalk still cannot be eliminated.

As shown in FIG. 1A, the light guide is in direct contact with thesilicon. This interface can cause undesirable backward reflection awayfrom the sensor. Conventional anti-reflection structures for imagesensors include the insertion of a nitride or oxynitride layer onlysolve reflection problem between the silicon and a thick oxideinsulator. This approach is not applicable when the interface is siliconand nitride.

BRIEF SUMMARY OF THE INVENTION

An image sensor pixel that includes a photoelectric conversion unitsupported by a substrate and an insulator adjacent to the substrate. Thepixel may have a plurality of cascaded light guides, wherein a portionof the cascaded lights guides is within the insulator and anotherportion extends above the insulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration showing a cross-section of two image sensorpixels of the prior art;

FIG. 1B is an illustration showing light cross-talk between adjacentpixels of the prior art;

FIG. 2 is an illustration showing a cross-section of two image sensorpixels of the present invention;

FIG. 3A is an illustration showing light traveling along an air gapbetween two color filters;

FIG. 3B is an illustration showing the redirection of light from the airgap into the color filters;

FIG. 3C is a graph of light power versus the distance along the air gap;

FIG. 3D is a graph of gap power loss versus gap width versus distancealong the air gap of widths 0.6 um and 1.0 um for three differentcolors;

FIG. 3E is a graph of maximal gap power loss versus gap width at a depthof 1.0 um;

FIG. 3F is a table of maximal gap power loss for different gap widths ata depth of 1.0 um;

FIG. 3G is a table of gap area as percentage of pixel area for differentgap widths and different pixel pitches;

FIG. 3H is a table of pixel power loss for different gap widths anddifferent pixel pitches;

FIG. 3I is a graph of pixel power loss versus pixel pitch for differentgap widths;

FIGS. 4A-L are illustrations showing a process used to fabricate thepixels shown in FIG. 3;

FIG. 5 is an illustration showing ray traces within the pixel of FIG. 2;

FIG. 6A is an illustration showing a pixel at a corner of the array;

FIG. 6B is an illustration showing light ray traces within the pixel ofFIG. 6A;

FIG. 7 is an illustration showing a top view of four pixels within anarray;

FIG. 8 is an alternate embodiment of the sensor pixels with ray tracing;

FIGS. 9A-M are illustrations showing a process used to fabricate thepixels shown in FIG. 8;

FIGS. 10A-H are illustrations showing a process to expose a bond pad;

FIG. 11 is an illustration showing an anti-reflection layer within thesensor;

FIGS. 12A-E are illustrations showing an alternate process to form ananti-reflection layer within the sensor;

FIG. 13A is a graph of transmission coefficient versus light wavelengthfor an anti-reflection layer;

FIG. 13B is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer;

FIG. 13C is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer;

FIGS. 14A-G are illustrations showing an alternate process to form twoanti-reflection layers within the sensor;

FIG. 15A is a graph of transmission coefficient versus light wavelengthfor a first anti-reflection layer on a left hand portion of FIG. 14G;

FIG. 15B is a graph of transmission coefficient versus light wavelengthfor a second anti-reflection layer shown on a right hand portion of FIG.14G.

DETAILED DESCRIPTION

Disclosed is an image sensor pixel that includes a photoelectricconversion unit supported by a substrate and an insulator adjacent tothe substrate. The pixel includes a light guide that is located withinan opening of the insulator and extends above the insulator such that aportion of the light guide has an air interface. The air interfaceimproves the internal reflection of the light guide. Additionally, thelight guide and an adjacent color filter, are constructed with a processthat optimizes the upper aperture of the light guide. Thesecharacteristics of the light guide eliminate the need for a microlens.

The pixel may include two light guides, one above the other. The firstlight guide is located within a first opening of the insulator adjacentto the substrate. The second light guide is located within a secondopening in a support film, which is eventually removed duringfabrication of the pixel. A color filter is located within the sameopening and is aligned with the second light guide. The second lightguide may be offset from the first light guide at the outer corners ofthe pixel array to capture light incident at a non-zero angle relativeto the vertical axis.

An air gap is created between neighboring color filters by removing thesupport film material adjacent to the filter. Air has a lower refractiveindex than the support film and enhances internal reflection within thecolor filter and the light guide. In addition, the air gap is configuredto “bend” light incident on the gap into the color filter and increasethe amount of light provided to the sensor.

Reflection at the silicon-light-guide interface is reduced by creating anitride film and a first oxide film below the first light guide. Asecond oxide film may be additionally inserted below the nitride film tobroaden the range of light frequencies for effective anti-reflection.The first oxide film can be deposited into an etched pit beforeapplication of the light-guide material. An alternate embodiment ofanti-reflection has all anti-reflection films formed before a pit isetched and an additional light guide etch-stop film above to protect theanti-reflection film from the pit etchant.

Referring to the drawings more particularly by reference numbers, FIGS.2, 4A-L, 5 and 6A-B show embodiments of two adjacent pixels in an imagesensor 100. Each pixel includes a photoelectric conversion 102 thatconverts photonic energy into electrical charges. The charges arecarried by a transfer electrode 104 located adjacent to each conversionunit 102. The electrodes 104 and conversion units 102 are formed on asubstrate 106. The sensor 100 also includes wires 108 that are embeddedin an insulating layer 110.

Each pixel has a first light guide 116. The first light guides 116 areconstructed with a refractive material that has a higher index ofrefraction than the insulating layer 110. As shown in FIG. 4B, eachfirst light guide 116 may have a sidewall 118 that slopes at an angle αrelative to a vertical axis. The angle α is selected to be less than90−asin(n_(insulating layer)/n_(light guide)) preferably 0, so thatthere is total internal reflection of light within the guide, whereinn_(insulating layer) and n_(light guide) are the indices of refractionfor the insulating layer material and light guide material,respectively. The light guides 116 internally reflect light from thesecond light guide 130 to the conversion units 102.

The second light guides 130 are located above first light guides 116 andmay be made from the same material as the first light guide 116. The topend of the second light guide 130 is wider than the bottom end, wherethe second light guide 130 meets the first light guide 116. Thus the gapbetween adjacent second light guides 130 at the bottom (henceforth“second gap”) is larger than at the top, as well as larger than the airgap 422 between the color filters 114B, 114G above the second lightguides 130. The second light guides 130 may be offset laterally withrespect to the first light guides 116 and/or the conversion unit 102, asshown in FIG. 6A, wherein the centerline C2 of the second light guide130 is offset from the centerline C1 of the first light guide 116 or ofthe photoelectric conversion unit 102. The offset may vary dependingupon the pixel position within an array. For example, the offset may begreater for pixels located at the outer portion of the array. The offsetmay be in the same lateral direction as the incident light to optimizereception of light by the first light guide. Offset second light guides130 capture more light that is incident at a nonzero angle relative tothe vertical axis. Effectively second light guide 130 and first lightguide 114 together constitute a light guide that takes different shapesat different pixels. The shape is optimized to the incident light rayangle at each pixel.

FIGS. 5 and 6B illustrate ray tracing for a pixel at the center of anarray and at a corner of the array, respectively. In FIG. 5, incidentlight rays come in vertically. The second light guides 130 are centeredto the first light guides 116. Both light rays a and b reflect oncewithin the second light guide 130 then enter the first light guide 116,reflects once (ray a) or twice (ray b) and then enter conversion units102. In FIG. 6B, the second light guides 130 are offset to the right,away from the center of the array, which is towards the left. Light rayc, which comes in from the left at an angle up to 25 degrees relative tothe vertical axis, reflects off the right sidewall of the second lightguide 130, hits and penetrate the lower-left sidewall of the same,enters the first light guide 116, and finally reaches conversion unit102. The offset is such that the first light guide 116 recaptures thelight ray that exits lower-left sidewall of second light guide 130. Ateach crossing of light guide sidewall, whether exiting the second lightguide or entering the first light guide, light ray c refracts in a waythat the refracted ray's angle to the vertical axis becomes less eachtime, enhancing propagation towards the photoelectric conversion unit.

Incorporating two separate lights guides, even if both use the samelight guide material, has a second advantage of reducing the etchingdepth for each light guide. Consequently, slope side wall etching iseasier to achieve with higher accuracy. It also makes deposition oflightguide material less likely to cause unwanted keyholes, which oftenhappen when depositing thin film into deep cavities, in this casecausing light to scatter from the light guide upon encountering thekeyholes.

Color filters 114 are located above the second light guides 130. Thesidewall upper portion at and adjacent to the color filters is morevertical than the rest of second lightguide. Viewing it another way,sidewalls of adjacent color filters facing each other are essentiallyparallel.

There is a first air-gap 422 between the color filters having a width of0.45 um or less. The depth of the first air-gap 422 is equal to 0.6 umor greater. The first air-gap has a width of preferably 0.45 um or less.An air gap with the dimensional limitations cited above causes the lightwithin the gap to be diverted into the color filters and eventually tothe sensors. Thus the percentage loss of light impinging on the pixeldue to passing through the gap (henceforth “pixel loss”) issubstantially reduced.

Light incident upon a gap between two translucent regions of higherrefractive indices become diverted to one or the other when the gap issufficiently narrow. In particular, light incident upon an air gapbetween two color filters diverts to one color filter or the other whenthe gap width is sufficiently small. FIG. 3A shows a vertical gapbetween two color filter regions filled with a lower refractive indexmedium, e.g. air. Incident light rays entering the gap and nearer onesidewall than the other is diverted towards and into the former, whereasthe rest are diverted towards and into the latter. FIG. 3B showswavefronts spaced one wavelength apart. Wavefronts travel at slowerspeed in higher refractive index medium, in this example the colorfilter having an index n of approximately 1.6. Thus the spacing betweenwavefronts in the gap, assuming air filled, is 1.6 times that of thecolor filter, resulting in the bending of wavefronts at the interfacebetween the color filter and air gap and causing the light rays todivert into the color filter. FIG. 3C is a graph of propagated lightpower P(z) along a vertical axis z of the air gap divided by theincident light power P(0) versus a distance z. As shown by FIG. 3C,light power decreases deeper into the gap for different gap widths, morerapidly for lesser gap widths on the order of one wavelength andconverges to be essentially negligible for a gap width of 0.4 timeswavelength or less at a depth of 1.5 times wavelength. From FIG. 3C, itis preferable to have a depth equal to at least 1 times the wavelengthof the longest wavelength of interest, which is 650 nm in thisembodiment for a visible light image sensor. At this depth, thepercentage of light power incident upon the gap and lost to the spacefurther below (henceforth “gap loss”) is less than 15%. The color filterthus needs to have thickness at least 1 time the wavelength in order tofilter incident light entering the gap to prevent unfiltered light frompassing on to light guides 130, 114 and eventually to the conversionunit 102. If the gap is filled with a transparent medium other than air,with refractive index n_(gap)>1.0, then presumably the gap would need tonarrow to 0.45 um/n_(gap) or less, since effectively distances in termsof wavelength remains the same but absolute distances are scaled by1/n_(gap).

For red light of wavelength in air of 650 nm, at a depth of 0.65 um(i.e. 1.0 time wavelength in air) the gap power flux attenuates to 0.15(15%) for a gap width of 0.6 time wavelength in air, i.e. 0.39 um.Attenuation reaches maximum at around 1 um of depth. Attenuation issteeper with depth for shorter wavelengths.

FIG. 3D shows the gap loss versus gap width W for 3 colors—blue at 450nm wavelength, green at 550 nm, and red at 650 nm—at depths of 0.6 umand 1.0 um, respectively. For a depth of 1.0 um, the highest gap lossamong the 3 colors and the maximal gap loss for gap widths of 0.2 um to0.5 um are plotted in FIG. 3E. Gap loss against gap width is tabulatedin FIG. 3F. In FIG. 3G, gap area as percentages of pixel areas istabulated against pixel pitch and gap width. Each entry (percentage gaparea) in the table of FIG. 3G is multiplied with the correspondingcolumn entry (i.e. gap loss) to give pixel loss as tabulated in FIG. 3H.FIG. 3I plots pixel loss versus pixel pitch for different gap widthsranging from 0.2 um to 0.5 um.

FIG. 3I shows that keeping gap width below 0.45 um would result in lessthan 8% pixel loss for pixel pitch between 1.8 um and 2.8 um—the rangeof pixel sizes for compact cameras and camera phones—for color filterthickness of 1.0 um. For less than 3%, a gap width below 0.35 um isneeded; for less than 1.5%, a gap width below 0.3 um; and for less than0.5%, a gap width below 0.25 um. FIG. 3I also shows that pixel loss isless for bigger pixels given the same gap width. Thus for pixels largerthan 5 um, the above guidelines result in at least halving the pixelloss.

Air interface may continue from the color filter sidewall along thesecond light guide sidewall and end above protection film 410, creatinga second air gap 424. The air interface between second air gap 424 andthe second light guide 130 enhances internal reflection for the secondlight guide 130.

A protection film 410 may be formed above insulating layer 110 ofsilicon nitride to prevent alkali metal ions from getting into thesilicon. Alkali metal ions, commonly found in color filter materials,can cause instability in MOS transistors. Protection film 410 also keepsout moisture. The protection film 410 may be made of silicon nitride(Si3N4) of thickness between 10,000 Angstroms and 4,000 Angstroms,preferably 7,000 Angstroms. If either first light guide 116 or secondlight guide 130 is made of silicon nitride, the protection film 410which is formed of silicon nitride is continuous across and above theinsulating layer 110 to seal the transistors from alkali metal ions andmoisture. If both first 116 and second 130 light guides are not made ofsilicon nitride, the protection film 110 may cover the top surface ofthe first light guide 116 to provide similar sealing or, alternatively,cover the sidewalls and bottom of first light guide 116.

First 422 and second 424 air gaps together form a connected opening toair above the top surface of the image sensor. Viewing this in anotherway, there exists a continuous air interface from the protection film410 to the top surfaces of the color filters 114B, 114G. In particular,there is an air-gap between the top surfaces 430 of the pixels. Theexistence of this opening during manufacture allows waste materialsformed during the forming of first air gap 422 and second air gap 424 tobe removed during the manufacture of the image sensor. If for somereason the first air-gap 422 is sealed subsequently using some plugmaterial, this plug material should have a refractive index lower thanthe color filter material so that (i) there is internal reflectionwithin the color filter, and (ii) light incident within the air-gap 422is diverted into the color filters 114B, 114G. Likewise if some fillmaterial fills the second air gap 424, this fill material needs to havea lower refractive index than the second light guide 130.

Together, the color filter 114 and light guides 130 and 116 constitute a“cascaded light guide” that guides light to the photoelectric conversionunit by utilizing total internal reflection at the interfaces withexternal media such as the insulator 110 and air gaps 422 and 424.Unlike prior art constructions, light that enters the color filter doesnot cross over to the color filter of the next pixel but can onlypropagate down to the second light guide 130.

A cascaded light guide further holds a second advantage over prior artthat uses opaque wall material between color filters. Incident lightfalling into the first air gap 422 between color filters 114B and 114Gis diverted to either one, thus no light is lost, unlike prior artpixels where light is lost in the gaps between the filters.

A first advantage of this color filter forming method over prior artmethods is that the color filter sidewall is not defined by thephotoresist and dye materials constituting the color filters. In priorart color filter forming methods, the color filter formed must producestraight sidewalls after developing. This requirement places a limit onthe selection of photoresist and dye material because the dye must notabsorb light to which the photoresist is sensitive, otherwise the bottomof the color filter will receive less light, resulting in color filterthat is narrower at its bottom than its top. The present color filterforming method forms the color filter sidewall by the pocket 210 etchedinto the support film 134, thus completing removing any constraints onthe choice of dye material, resulting in a cheaper process.

A second advantage over prior art color filter forming methods is thatgap spacing control is uniform between all pixels, and highly accurateat low cost. The gap spacing is a combination of the line-width in thesingle lithography step that etches the openings in the support film,plus the control of sideway etching during dry etch. If such gaps wereto be created by placing 3 color filters of different colors at 3different lithography steps as in the prior art, uniformity of gapwidths is impossible, the lithography steps become expensive, andsidewall profile control becomes even more stringent.

A cascaded light guide wherein a color filter and a light guide areformed in the same opening in the support film (henceforth “self-alignedcascaded light guide”) has an advantage over prior art in that there isno misalignment between the color filter 114 and the second light guide130. The color filter 114 has sidewalls that are self-aligned tosidewalls of the second light guide 130.

FIGS. 4A-L show a process for forming the image sensor 100. The sensormay be processed to a point wherein the conversion units 102 andelectrodes 104 are formed on the silicon substrate 106 and the wires 108are embedded in the insulator material 110 as shown in FIG. 4A. Theinsulator 110 may be constructed from a low refractive index materialsuch as silicon dioxide. The top of the insulator 110 can be flattenedwith a chemical mechanical polishing process (“CMP”).

As shown in FIG. 4B, insulating material may be removed to form lightguide openings 120. The openings 120 have sloping sidewalls at an angleα. The openings 120 can be formed, by example, using a reactive ionetching (“RIE”) process. For silicon oxide as the insulating material, asuitable etchant is CF₄+CHF₃ in a 1:2 flow ratio, carried in Argon gasunder 125 mTorr, 45° C. The sidewall angle may be adjusted by adjustingthe RF power between 300 W and 800 W at 13.56 MHz.

FIG. 4C shows the addition of light guide material 122. By way ofexample, the light guide material 122 can be a silicon nitride that hasan index of refraction greater than the refractive index of theinsulating material 110. Additionally, silicon nitride provides adiffusion barrier against H₂O and alkali metal ions. The light guidematerial can be added for example by plasma enhanced chemical vapordeposition (“PECVD”).

The light guide material may be etched so that the material covers theinsulator. This seals the conversion unit and electrodes duringsubsequent processes. Alternatively, if the first light guide material122 is not silicon nitride, then a silicon nitride film may be depositedon top of light guide material 122 such that silicon nitride film sealsthe conversion unit and electrodes. This silicon nitride film may bebetween 10,000 Angstroms and 4,000 Angstroms thick, preferably 7,000Angstroms.

A shown in FIG. 4D a support film 134 is formed on top of the siliconnitride. The support film 134 may be silicon oxide.

In FIG. 4E, the support film is etched to form openings. The openingsmay include sidewalls 136 that slope at an angle β. The angle β isselected so that β<90−asin(1/n2_(light guide)) where n2_(light guide) isthe index of refraction of the second light guide material 130, suchthat there is a total internal reflection within the second light guides130. Incorporating two separate lights guides reduces the etching depthfor each light guide. Consequently, slope side wall etching is easier toachieve with higher accuracy. The support film 134 and second lightguides 130 can be made from the same materials and with the sameprocesses as the insulating layer 110 and first light guides 116,respectively.

As shown in FIG. 4E the sidewall may have a vertical portion and asloped portion. The vertical portion and sloped portion can be achievedby changing the etching chemistry or plasma conditions during theetching process. The etch recipe during the vertical portion etch isselected to be favorable for forming the vertical sidewall 162, thenswitched to a recipe favorable for forming the sloped sidewall.

FIG. 4F shows the addition of light guide material. By way of example,the light guide material can be a silicon nitride. The light guidematerial can be added for example by plasma enhanced chemical vapordeposition (“PECVD”). FIG. 4G shows each second light guide 130 has apocket 210. The pockets 210 are separated by a sidewall 212 of thesupport film 134. As shown in FIG. 4H, a color film material 114B havinga dye of a particular color is applied to fill the pockets 210 andextends above the support film 134. In this example, the color materialmay contain blue dye. Color filter material is typically made ofnegative photoresist, which forms polymers that when exposed to lightbecomes insoluble to a photoresist developer. A mask (not shown) isplaced over the material 114 with openings to expose areas that are toremain while the rest is etched away. FIG. 4I shows the sensor after theetching step. The process can be repeated with a different colormaterial such as green to create color filters for each pixel as shownin FIG. 4J.

The pockets 210 provide an alignment feature to align the color filtermaterial with the second light guide 130. The pockets 210 may be largerthan the corresponding mask openings. The etchant may be adjusted tocreate more sideways (i.e. isotropic) etch and reduce the thickness ofthe side wall 212. This results in an optimal second light guide openingfor a given pixel pitch.

As shown in FIG. 4K the color filter is etched to expose the supportwall 212 of the support film 134. A portion of the support film is thenremoved as shown in FIG. 4L so that there is an air/material interfacefor the color filters 114 and second light guide 130. As shown in FIG.5, light internally reflects along the color filters 114 and lightguides 130 and 116. The color filter 114 has a higher refractive indexthan air so that the filter 114 provides internal reflection. Likewise,the second light guide 130 has an air interface which improves theinternal reflection properties of the guide. FIG. 7 is a top viewshowing four pixels 200 of a pixel array. For embodiments that includeboth first and second light guides the area B may be the area of thesecond light guide top surface and the area C represents the area of thefirst light guide bottom surface.

FIG. 8 shows an alternate embodiment wherein the lights guides are madefrom the same material. A process for fabricating the alternateembodiment is shown in FIGS. 9A-M. The process is similar to the processshown in FIGS. 4A-L, except the opening for the first light guide isformed after the opening for the second light as shown in FIG. 9F. Bothlight guides are formed in the same step shown in FIG. 9G.

FIGS. 10A-H show a process to expose bond pads 214 of the image sensor.An opening 216 is formed in a first insulator material 110 that covers abond pad 214 as shown in FIGS. 10A-B. As shown in FIGS. 10C-D the firstlight guide material 116 is applied and a substantial portion of thematerial 116 is removed. The support film material 134 is applied and acorresponding opening 218 is formed therein as shown in FIGS. 10E-F. Thesecond light guide material 130 is applied as shown in FIG. 10G. Asshown in FIG. 10H a maskless etch step is used to form an opening 220that exposes the bond pad 214. The etchant preferably has acharacteristic that attacks light guide material 110 and 130 (e.g.silicon nitride) much faster than the insulator material 110 and 134(silicon oxide) and color filter 114 (photoresist). Dry etch in CH₃F/O₂has 10× greater etch rate on silicon nitride than on color filter orsilicon oxide.

FIG. 11 shows an embodiment wherein an anti-reflection (AR) stackcomprising a top AR film 236, second AR film 234, and a third AR film236 cover the conversion units 102. The anti-reflection stack improvesthe transmission of light from the first light guide 116 to theconversion units 102. Members of the AR stack together may constitutelayer 230 that also blanket the substrate 106, conversion units 102 andelectrodes 104 to protect the elements from chemical pollutants andmoisture. For example, the second AR film 234 may be a contact etch-stopnitride film common in CMOS wafer fabrication, constituting layer 230.The third AR film 232 may be silicon oxide. This silicon oxide film maybe a gate insulating film under the gate electrode 114, or the spacerliner oxide film that runs down the side of the gate electrode 114between the gate and the spacer (not shown) in common deep submicronCMOS processes, a silicide-blocking oxide film (commonly used to preventstress-induced leakage in image sensor pixels) deposited before contactsilicidation to block contact siliciding, or a combination thereof.Using an existing silicon nitride contact etch-stop film as part of theAR stack provides cost savings. The same contact etch-stop film alsofunctions to stop the etch of the opening in insulator 110 forfabrication of the light guide. Finally, the top AR film 236 is formedin the opening in the insulator 110 prior to filling the opening withlight guide material.

The top AR film 236 has a lower refractive index than the light guide116. The second film 234 has a higher refractive index than the top film236. The third film 232 has a lower refractive index than the secondfilm 234.

The top AR film 236 may be silicon oxide or silicon oxynitride, with athickness between 750 Angstrom and 2000 Angstrom, preferably 800Angstrom. The second film 234 may be silicon nitride (Si₃N₄), with athickness between 300 Angstrom and 900 Angstrom, preferably 500Angstrom. The third film may be silicon oxide or silicon oxynitride(SiOxNy, where 0<x<2 and 0<y<4/3), with a thickness between 25 Angstromand 170 Angstrom, preferably 75 Angstrom.

The anti-reflection structure shown in FIG. 11 can be fabricated byfirst forming the third AR film 232 and the second AR film 234 over thesubstrate, respectively. The insulator 110 is then formed on the secondAR film 234. The silicon nitride film is formed on the first insulator110 in a manner that covers and seals the insulator and underlyinglayers to form a protection film 410 with a thickness between 10,000Angstrom and 4,000 Angstrom, preferably 7,000 Angstrom. The support film134 is formed on the light guide material.

The support film 134 is masked and a first etchant is applied to etchopenings in the support film 134. The first etchant is chosen to havehigh selectivity towards the silicon nitride protection film material.For example, if support film 134 is HDP silicon oxide, the first etchantmay be CHF₃, which etches HDP silicon oxide 5 times as fast as siliconnitride. A second etchant is applied to etch through the silicon nitrideprotection film material to deepen the openings. The second etchant maybe CH₃F/O₂. The first etchant is again applied to etch the first silicondioxide insulator material but not the silicon nitride contact etch-stopfilm 234. The silicon nitride acts as an etchant stop that defines thebottom of the opening. The top AR film 236 is then formed in theopening. An etchant can be applied to etch away the top AR film materialthat extends along the sidewalls of the opening, for example by dry etchusing the first etchant and holding the wafer substrate at a tilt angleand rotated about the axis parallel to the incoming ion beam. Lightguide material is then formed in the openings. The color filter can beformed over the light guide and a portion of the support film can beetched to create the structure shown in FIG. 5.

FIGS. 12A-E show a process for fabricating another embodiment ofanti-reflection between the light guide and substrate. In thisembodiment an etch-stop film 238 is interposed between the light guideand the anti-reflection stack comprising the top AR film 236, second ARfilm 234, and third AR film 232. The light guide etch-stop film 238 maybe formed of the same material as the light guide, and may be siliconnitride, with a thickness between 100 Angstrom and 300 Angstrom,preferably 150 Angstrom. Forming the AR stack in this embodiment has anadvantage of more precise control of the thickness of the second ARfilm, at the expense of one more deposition step and the slight addedcomplexity of etching through a oxide-nitride-oxide-nitride-oxide stackinstead of oxide-nitride-oxide stack. The previous embodiment uses thesecond AR film 234 as a light guide etch stop and loses some ofthickness to the final step of insulator pit etch over-etch.

As shown in FIGS. 12A-B, the third 232 and second 234 AR films areapplied to the substrate 106 and then a top AR film 236 is applied ontothe second AR film 234, followed by a light guide etch-stop film 238made of silicon nitride. As shown in FIG. 12C, the insulator layer 110and wiring electrodes 108 are formed above the AR films and light guideetch-stop film. FIG. 12D shows an opening formed in insulator 110,stopping at the top of the light guide etch-stop film 138. FIG. 12Eshows the opening filled with light guide material.

FIG. 13A is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer of FIG. 11 and FIG. 12E, for nominal topAR film (oxide) of 800 Angstroms thick, and varied +/−10%, whereassecond AR film (nitride) is 500 Angstroms thick and third AR film(oxide) is 75 Angstroms thick. It exhibits steep decline in the violetcolor region (400 nm to 450 nm). The nominal thicknesses of the AR filmsconstituting the AR layer are chosen to position the maximum of thetransmission curve in the blue color region (450 nm to 490 nm) thangreen color region (490 nm to 560 nm) so that any shift in filmthicknesses due to manufacturing tolerance would not result intransmission coefficient falloff much more in violet than in red colorregion (630 nm to 700 nm).

FIG. 13B is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer of FIG. 11 and FIG. 12E, for nominalsecond AR film (nitride) of 500 Angstroms thick, and varied +/−10%.

FIG. 13C is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer of FIG. 11 and FIG. 12E, for nominal thirdAR film (nitride) of 75 Angstroms thick, and varied +/−10%.

FIGS. 14A-G show a process for fabricating another embodiment ofanti-reflection between the light guide and substrate to provide twodifferent AR layers that each optimize for a different color region.Third and second AR film 232 and 234 are provided over the photoelectricconversion unit 201 in FIG. 14A, similar to the embodiment shown in FIG.12A. In FIG. 14A, the top AR film 236 is deposited to the thickness ofthicker top AR film 236 b shown in FIG. 14B. Subsequently a lithographymask (not shown) is applied to create mask openings over the pixels thatuse the thinner top AR film 236 a. An etch step is applied to thin thetop AR film 236 under the mask opening to the smaller thickness of topAR film 236 a in FIG. 14B. Subsequent steps are identical to FIGS.12B-E.

FIG. 15A is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layers of FIG. 14G for a nominal thinner top ARfilm 236 a of nominal thickness 0.12 um, a second AR film 234 of nominalthickness 500 Angstroms, and a third AR film 232 of nominal thickness 75Angstroms. This graph peaks in the green color region at approximately99%, and drops gently to approximately 93% at the center of the redcolor region. This graph shows that the top AR film 236 a can be used atred pixels as well as green pixels.

FIG. 15B is a graph of transmission coefficient versus light wavelengthfor the anti-reflection layer of FIG. 14G for a top AR film 236 b ofnominal thickness 0.20 um, a second AR film 234 of nominal thickness 500Angstroms, and a third AR film 232 of nominal thickness 75 Angstroms.This graph peaks in two separate color regions, viz. purple and red.This graph shows that the top AR film 236 b can be used at blue pixelsand red pixels.

A pixel array can use top the AR film 236 a for green pixels only whilethe top AR film 236 b for both blue and red pixels. Alternately, thepixel array can use the top AR film 236 a for both green and red pixelswhile the top AR film 236 b is used for blue pixels only.

Another embodiment to provide two different AR layers that eachoptimizes for a different color region can be provided by creatingdifferent second AR film thicknesses while keeping the same top AR filmthickness. Two different thicknesses are determined, one for each colorregion. The second AR film is first deposited to the larger thickness.Subsequently a lithography mask is applied to create a mask opening overthe pixels that uses the smaller second AR film thickness. An etchingstep is applied to thin the second AR film under the mask opening to thesmaller thickness. Subsequent steps are identical to FIGS. 12B-E.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

1. An image sensor, comprising: a substrate; a photoelectric conversion unit in said substrate; a light guide, said light guide having a light-guide refractive index; and, an anti-reflection stack under said light guide and on said substrate, said anti-reflection stack disposed to transmit a light from said light guide to said photoelectric conversion unit, consisting essentially of: a third film on said substrate, said third film having a third refractive index lower than said light-guide refractive index; a second film on said third film, said second film having a second refractive index higher than said third refractive index; and a top film on said second film, said top film having a first refractive index lower than said light-guide refractive index and said second refractive index.
 2. The image sensor of claim 1, wherein said top film comprises silicon oxide and has a thickness within a range of 800 Angstroms±10%.
 3. The image sensor of claim 1, wherein said second film comprises silicon nitride.
 4. The image sensor of claim 1, wherein said third film comprises silicon oxide and has a thickness within a range of 75 Angstroms±10%.
 5. The image sensor of claim 1, wherein said light guide comprises silicon nitride.
 6. An image sensor, comprising: a substrate; a photoelectric conversion unit in said substrate; a light guide, said light guide having a light-guide refractive index; a light-guide etch-stop film under said light guide; and, an anti-reflection stack under said light-guide etch-stop film and on said substrate, said anti-reflection stack disposed to transmit a light from said light guide to said photoelectric conversion unit, consisting essentially of: a third film on said substrate, said third film having a third refractive index lower than said light-guide refractive index; a second film on said third film, said second film having a second refractive index higher than said third refractive index; and a top film on said second film, said top film having a first refractive index lower than said light-guide refractive index and said second refractive index.
 7. The image sensor of claim 6, wherein said top film comprises silicon oxide and has a thickness within a range of 800 Angstroms±10%.
 8. The image sensor of claim 6, wherein said second film is also a contact etch stop film.
 9. The image sensor of claim 6, wherein said third film comprises silicon oxide and has a thickness within a range of 75 Angstroms±10%.
 10. A method for forming an image sensor, comprising: providing a substrate; forming a photoelectric conversion unit in said substrate; forming a third film on said substrate and over said photoelectric conversion unit, said third film having a third refractive index; forming a second film on said third film, said second film having a second refractive index higher than said third refractive index; forming a top film on said second film, said top film having a first refractive index lower than said second refractive index; and, forming a light guide on said top film, said light guide comprising a light guide material having a light guide refractive index higher than said first and third refractive indices.
 11. The method of claim 10, wherein said second film comprises silicon nitride.
 12. The method of claim 10, wherein said second film has a thickness within a range of 500 Angstroms±10%.
 13. The method of claim 10, wherein said second film has a thickness between 300 Angstroms and 900 Angstroms.
 14. The method of claim 10, wherein said second film is also a contact etch-stop film.
 15. The method of claim 10, wherein said top film has a thickness between 750 Angstroms and 2000 Angstroms.
 16. The method of claim 15, wherein said top film comprises a silicon oxide.
 17. The method of claim 15, wherein said top film comprises a silicon oxynitride.
 18. The method of claim 10, wherein said third film comprises a silicon oxide or a silicon oxynitride.
 19. The method of claim 10, wherein a transmission coefficient of light from the light guide to the substrate across the top, second and third films peaks below 490 nm of wavelength-of-light-in-air.
 20. The method of claim 10, further comprising: forming an insulating film on the second film; and, forming an opening in the insulating film to expose the second film, after which the top film is formed in the opening.
 21. A method for forming an image sensor, comprising: providing a substrate; forming a photoelectric conversion unit in said substrate; forming a third film on said substrate and over said photoelectric conversion unit, said third film having a third refractive index; forming a second film on said third film, said second film having a second refractive index higher than said third refractive index; forming a top film on said second film, said top film having a first refractive index lower than said second refractive index; forming a light-guide etch-stop film on said top film; and, forming a light guide on said light-guide etch-stop film, said light guide comprising a light guide material having a light guide refractive index higher than said first and third refractive indices.
 22. The method of claim 21, wherein said second film comprises a silicon nitride.
 23. The method of claim 21, wherein said second film has a thickness between 300 Angstroms and 900 Angstroms.
 24. The method of claim 21, wherein said second film is also a contact etch-stop
 25. The method of claim 21, wherein said top film has a thickness between 750 Angstroms and 2000 Angstroms.
 26. The method of claim 25, wherein said top film comprises a silicon oxide.
 27. The method of claim 25, wherein said top film comprises a silicon oxynitride.
 28. The method of claim 21, wherein said third film comprises a silicon oxide or a silicon oxynitride.
 29. The method of claim 21, wherein a transmission coefficient of light from the light guide to the substrate across the top, second and third films peaks below 490 nm of wavelength-of-light-in-air.
 30. The method of claim 21, wherein a transmission coefficient of light from the light guide to the substrate across the top, second and third films has a trough between 500 nm and 600 nm of wavelength-of-light-in-air.
 31. The method of claim 21, further comprising: etching back at least one of the top and second films.
 32. The image sensor of claim 1, wherein a transmission coefficient of light from the light guide to the substrate across the top, second and third films peaks below 490 nm of wavelength-of-light-in-air.
 33. The image sensor of claim 6, wherein a transmission coefficient of light from the light guide to the substrate across the top, second and third films peaks below 490 nm of wavelength-of-light-in-air.
 34. The image sensor of claim 6, wherein said light-guide etch-stop film comprises a silicon nitride.
 35. The image sensor of claim 1, wherein said top film has a thickness between 750 Angstroms and 2000 Angstroms.
 36. The image sensor of claim 35, wherein said top film comprises a silicon oxide.
 37. The image sensor of claim 35, wherein said top film comprises a silicon oxynitride.
 38. The image sensor of claim 6, wherein said top film has a thickness between 750 Angstroms and 2000 Angstroms.
 39. The image sensor of claim 38, wherein said top film comprises a silicon oxide.
 40. The image sensor of claim 38, wherein said top film comprises a silicon oxynitride.
 41. The image sensor of claim 6, further comprising: another anti-reflection stack below another light guide that shares the light-guide refractive index, said other anti-reflection stack disposed to transmit another light from said other light guide to another photoelectric conversion unit in said substrate, said other anti-reflection stack consisting essentially of: another third film on said substrate, said other third film has same thickness, composition and refractive index as said third film; another second film on said other third film, said other second film has same composition and refractive index as said second film; and, another top film on said other second film, said other top film has same composition and refractive index as said top film, wherein either said other top film has a lesser thickness than said top film and said other second film has same thickness as said second film or said other second film has a lesser thickness than said second film and said other top film has same thickness as said top film.
 42. The image sensor of claim 41, wherein, between said anti-reflection stack and said other anti-reflection stack, one has a peak of transmission coefficient of light below 500 nm of wavelength-of-light-in-air whereas the other has a peak of transmission coefficient of light above 500 nm of wavelength-of-light-in-air.
 43. The image sensor of claim 41, wherein, between said anti-reflection stack and said other anti-reflection stack, one is at a green pixel whereas the other is at a blue pixel. 